1. Field of the Invention
The present invention is in the area of novel 1,4-benzodiazepines and salts thereof, their syntheses, and their use as inhibitors of MDM2 and HDM2 oncoproteins.
2. Related Art
This invention relates to compounds that bind to the human protein HDM2 and interfere with its interaction with other proteins, especially the tumor suppressor protein p53. HDM2 is the expression product of hdm2, an oncogene that is overexpressed in a variety of cancers, especially soft tissue sarcomas (Momand, J., et al., Nucl. Acids Res. 26:3453–3459 (1998)).
p53 is a transcription factor that plays a pivotal role in the regulation of the balance between cell proliferation and cell growth arrest/apoptosis. Under normal conditions, the half-life of p53 is very short, and consequently the level of p53 in cells is low. However, in response to cellular DNA damage, cellular stress, or other factors, levels of p53 increase. This increase in p53 levels in turn increases the transcription of a number of genes which induces the cell to either arrest growth or undergo apoptosis (i.e., controlled cell death). The function of p53 is to prevent the uncontrolled proliferation of cells and thus protect the organism from the development of cancer (for a review, see Levine, A. J., Cell 88:323–331 (1997)).
p53 is a latent and short-lived transcription factor which is induced by, and is an integration point for, a range of cellular stresses including DNA damage, UV damage, spindle damage, hypoxia, inflammatory cytokines, viral infection, activated oncogenes, and ribonucleotide depletion. Activation of p53 mediates a change in the balance of gene expression such that expression of many genes involved in proliferation is repressed while a range of genes involved in growth arrest (such as p21WAF1 and GADD45), repair (such as p53RE) and apoptosis (such as Bax, Killer/DR5 and PIGs) is activated. The biological outcome of p53 activation (whether permanent or transient growth arrest or apoptosis) is dependent on several factors including the type and strength of the inducing stress, and the type of cell or tissue.
p53 and MDM2 exist in a negative regulatory feedback loop in which p53 stimulates transcription of the mdm2 gene while MDM2 binds to p53 and targets it for degradation by the 26S proteosome. The key element in the p53induction process is disruption of the p53-MDM2 complex which permits p53 to accumulate in the nucleus. This mechanism appears to be common to all of the pathways by which p53 becomes activated, although recent evidence has indicated that there is considerable variation in the molecular events by which this is actually achieved.
Inactivation of the p53 tumor suppressor is a frequent event in human neoplasia. The inactivation can occur by mutation of the p53 gene or through binding to viral or cellular oncogene proteins, such as the SV40 large T antigen and MDM2. While the mechanism through which wild-type p53 suppresses tumor cell growth is as yet poorly defined, it is clear that one key feature of the growth suppression is the property of p53 to act as a transcription factor (Farmer, G., et al., Nature 358: 83–86 (1992); Funk, W. D. et al., Mol. Cell. Biol. 12: 2866–2871 (1992); Kern, S. E., et al., Science 256:827–830 (1992)). Currently, considerable effort is being made to identify growth control genes that are regulated by p53 binding to sequence elements near or within these genes. A number of such genes have been identified. In cases such as the muscle creatine kinase gene (Weintraub, H., et al., Proc. Natl. Acad. Sci. U.S.A., 88:4570–4571 (1991); Zambetti, G. P., et al., Genes Dev. 6:1143–1152 (1992)) and a GLN retroviral element (Zauberman, A., et al., EMBO J. 12:2799–2808 (1993)), the role these genes might play in the suppression of growth control is unclear. Yet there are other examples, namely mdm2 (Barak, Y., et al. EMBO J. 12:461–468 (1993); Wu, X., et al., Genes Dev. 7:1126–1132(1993)) GADD 45 (Kastan, M. B., et al., Cell 71:587–597(1992)) and WAF1 or CIP1 (El-Beiry, W. S., et al., Cell 75:817–825 (1993); Harper, J. W., et al., Cell 75:805–816 (1993)), where their involvement in the regulation of cell growth is better understood.
mdm2, a known oncogene, was originally found on mouse double minute chromosomes (Cahilly-Snyder., L., et al., Somatic Cell Mol. Genet. 13:235–244 (1987)). Its protein product was subsequently found to form a complex with p53, which was first observed in a rat fibroblast cell line (Clone 6) previously transfected with a temperature sensitive mouse p53 gene (Michalovitz, D., et al., Cell 62:671–680 (1990)). The rat cell line grew well at 37° C. but exhibited a G1 arrest when shifted down to 32° C., which was entirely consistent with an observed temperature dependent switch in p53 conformation and activity. However, the p53-MDM2 complex was only observed in abundance at 32° C., at which temperature p53 was predominantly in a functional or “wild-type” form (Barak, Y. et al., EMBO J. 11:2115–2121 (1992) and Momand, J., et al., Cell 69:1237–1245 (1992)). By shifting the rat cell line down to 32° C. and blocking de novo protein synthesis it was shown that only “wild-type” p53 induced expression of the mdm2 gene, thereby accounting for the differential abundance of the complex in terms of p53 transcriptional activity (Barak, Y., et al., EMBO J. 12:461–468 (1993)). The explanation was further developed by the identification of a DNA binding site for wild-type p53 within the first intron of the mdm2 gene (Wu, X., et al., Genes Dev. 7:1126–1132 (1993)). Reporter constructs employing this p53 DNA binding site revealed that they were inactivated when wild-type p53 was co-expressed with MDM2.
This inhibition of the transcriptional activity of p53 may be caused by MDM2 blocking the activation domain of p53 and/or the DNA binding site. Consequently, it was proposed that mdm2 expression is autoregulated, via the inhibitory effect of MDM2 protein on the transcriptional activity of wild-type p53. This p53-mdm2 autoregulatory feedback loop provided a novel insight as to how cell growth might be regulated by p53. Up to a third of human sarcomas are considered to overcome p53-regulated growth control by amplification of the hdm2 gene (the human homologue of mdm2) (Oliner, J. D., et al., Nature 358:80–83 (1992)). Hence, the interaction between p53 and HDM2 represents a key potential therapeutic target. One mechanism by which MDM2 can promote tumorogenesis is by its inhibitory action on p53. The tumor suppressor functions of p53 control a pivotal checkpoint in the control of cell cycling (reviewed in Levine, A. J., Cell 88:323–331 (1997)). p53 is a transcription factor for a number of proteins that cause cell cycle arrest or cell death by apoptosis. The level and transcriptional activity of p53 are increased by damage to cellular DNA. The MDM2 protein inhibits p53 function by binding to an amphipathic N-terminal helix of p53, abrogating the interaction of p53 with other proteins and its transactivation activity. The interaction with MDM2 also targets p53 for ubiquitin dependent protein degradation. MDM2 exhibits p53 independent effects on cell cycling as well, possibly by direct interaction with some of the downstream effectors such as pRB and EF2 (Reviewed in Zhang, R. and Wang, H., Cur. Pharm. Des. 6:393–416 (2000)).
Blocking HDM2 from binding p53 would be therapeutically useful in restoring cell cycle control to cells that overexpress HDM2 as a front line cancer treatment. More generally, inhibition of HDM2 may increase the effectiveness of chemotherapy and radiation in p53 normal cancers by enhancing apoptosis and growth arrest signaling pathways.
A need continues to exist for potent, small molecules that inhibit the interactions between HDM2 and p53.